The chunk size for a specific collection is defined according to the following rules hierarchy:

1. The collection specific chunk size maxChunkSizeBytes field in config.collection if set
2. The global chunk size setting in config.settings.chunkSize if set
3. The default global chunk size setting

Nevertheless, such criteria aren't respected when a chunk is being migrated out of a shard (more specifically, the wrong value may be taken into consideration when the "end of the catch up phase" condition is evaluated).